A gel filtration technique has been used for analysing the interaction of testosterone and/or its metabolites with macromolecules in the ventral prostate, rectus abdomonis muscle, liver and kidney from castrated adult male rats. Following administration of (1,2-3H)-testosterone, association of radioactivity with soluble macromolecules was demonstrated in the ventral prostate but not in the other organs analysed. A similar binding was observed when slices of ventral prostate were incubated in vitro with testosterone. When 105 000 × g supernatants from the ventral prostate were incubated with (1,2-3H)-testosterone at 37° C, radioactivity was also recovered associated with macromolecules. No association could be detected when the incubation was performed at 0° C.